I've used it since my pup was small and haven't had a problem with it. No sickness, no lethargy, no dry skin. I have heard of more natural ways to treat for fleas such as diatomaceous earth, but I have not used it myself and cannot speak to its effectiveness. While the Trifexis is a pesticide, I would rather expose my dog to a pesticide that has a minimal chance of affecting his health than go without or with something less effective and raise his risk of dealing with the pain of heartworm. At least, that's my view on it.
And it can always also depend on the dog. Just as you'll probably hear about dogs getting sick on Trifexis but doing fine with a spot-on, I'm sure you'll hear about dogs getting sick off the spot-ons but doing great on a pill like Trifexis. With any new medication, my biggest piece of advice is to monitor your dog closely the first couple times you use it to make sure it's right for your dog.